1. Field
The present disclosure relates generally to manufacturing composite structures and, in particular, to manufacturing composite structures for aircraft. Still more particularly, the present disclosure relates to a method and apparatus for manufacturing composite structures using mandrels.
2. Background
Aircraft are being designed and manufactured with greater and greater percentages of composite materials. Composite materials are used in aircraft to decrease the weight of the aircraft. This decreased weight improves performance features such as payload capacities and fuel efficiencies. Further, composite materials provide longer service life for various components in an aircraft.
Composite materials are tough, light-weight materials created by combining two or more functional components. For example, a composite material may include reinforcing fibers bound in a polymer resin matrix. The fibers may be unidirectional or may take the form of a woven cloth or fabric. The fibers and resins are arranged and cured to form a composite material.
Composite materials may be used to create aerospace composite structures. For example, composite materials may be used to create a fuselage, a wing, or other structures for the aircraft.
In manufacturing composite structures, layers of composite material are typically laid up on a tool. The layers may be comprised of fibers in sheets. These sheets may take the form of fabrics, tape, tows, or other suitable forms. In some cases, resin may be infused or pre-impregnated into the sheets. These types of sheets are commonly referred to as prepreg.
The different layers of prepreg may be laid up in different orientations and different numbers of layers may be used depending on the thickness of the composite structure being manufactured. These layers may be laid up by hand or using automated lamination equipment such as a tape laminating machine or a fiber placement system.
After the different layers of composite material have been laid up on the tool, the layers of composite material may be consolidated and cured upon exposure to temperature and pressure, thus forming the final composite structure. With a wing, stringers may be associated with a composite skin to form the wing in a number of different ways. For example, the stringers may be cured and trimmed. The stringers may then be bonded to an uncured skin. In another example, uncured stringers may be co-bonded to the cured skin. In yet another illustrative example, uncured stringers may be cured together with an uncured skin. This type of process of curing all components together at the same time is called co-cure.
Curing a composite structure involves applying heat and, often times, also includes applying pressure. The pressure may be applied by applying a vacuum to a bag enclosing the composite structure. With respect to stringers, mandrels are used with vacuum bags to define a shape for the stringers when curing the composite materials for the stringers. The shapes may include a hat shape, an I beam, or other suitable shapes. Additionally, the mandrel also defines the stringer configuration and shape, and reduces wrinkles on the stringers.
As the length of the wing increases, the length of the stringers also increases. For example, a stringer may be about 90 feet or longer.
Increasing the length of the stringers results in more time to manufacture the stringer. For example, the time to bag the stringer for curing increases. With a stringer that is about 90 feet long, the time needed to install a mandrel and bag the stringer for curing may be about 4 hours for each stringer. Thus, the time for manufacturing a wing may be greater than desired. Additionally, this increase of time for processing stringers results in a need for additional mandrels and other tools. As a result, the tooling cost for manufacturing a wing may increase more than desired.
Therefore, it would be desirable to have a method and apparatus that take into account at least some of the issues discussed above, as well as other possible issues. For example, it may be desirable to have a method and apparatus that reduce the time needed to install a mandrel for a stringer and to bag the stringer.